EUV photolithography system fuel source and methods of operating the same

ABSTRACT

Impurities in a liquefied solid fuel utilized in a droplet generator of an extreme ultraviolet photolithography system are removed from vessels containing the liquefied solid fuel. Removal of the impurities increases the stability and predictability of droplet formation which positively impacts wafer yield and droplet generator lifetime.

BACKGROUND Technical Field

The present disclosure relates to the field of photolithography. Thepresent disclosure relates more particularly to extreme ultravioletphotolithography.

Description of the Related Art

There has been a continuous demand for increasing computing power inelectronic devices including smart phones, tablets, desktop computers,laptop computers and many other kinds of electronic devices. Integratedcircuits provide the computing power for these electronic devices. Oneway to increase computing power in integrated circuits is to increasethe number of transistors and other integrated circuit features that canbe included for a given area of semiconductor substrate.

The features on an integrated circuit die are produced, in part, withthe aid of photolithography. Traditional photolithography techniquesinclude generating a mask outlining the shape of features to be formedon an integrated circuit die. A photolithography light source irradiatesthe integrated circuit die through the mask. The size of the featuresthat can be produced via photolithography of the integrated circuit dieis limited, in part, on the lower end, by the wavelength of lightproduced by the photolithography light source. Smaller wavelengths oflight can produce smaller feature sizes.

Extreme ultraviolet light is used to produce particularly small featuresdue to the relatively short wavelength of extreme ultraviolet light. Forexample, extreme ultraviolet light is typically produced by irradiatingdroplets of selected materials from a droplet generator with a laserbeam. The energy from the laser causes the droplets to enter a plasmastate. In the plasma state, the droplets emit extreme ultraviolet light.The extreme ultraviolet light travels toward a collector with anelliptical or parabolic surface. The collector reflects the extremeultraviolet light onto the photolithography target.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a photolithography system, according to oneembodiment.

FIG. 2 is a block diagram of a photolithography system, according to oneembodiment.

FIG. 3 is a schematic top view of droplet generator in accordance withan embodiment of the present disclosure.

FIG. 4 is a schematic view of a delivery system for providing liquidfuel to a droplet generator in accordance with an embodiment of thepresent disclosure.

FIG. 5 is a schematic view of a vessel for containing a liquid fuel inaccordance with an embodiment of the present disclosure.

FIG. 6 is a schematic view of another vessel for containing a liquidfuel in accordance with an embodiment of the present disclosure.

FIG. 7 is a flow diagram of a method in accordance with an embodiment ofthe present disclosure.

FIG. 8 is a flow diagram of a method in accordance with an embodiment ofthe present disclosure.

DETAILED DESCRIPTION

In the following description, specific dimensions and materials aregiven by way of example for various embodiments. Those of skill in theart will recognize, in light of the present disclosure, that otherdimensions and materials can be used in many cases without departingfrom the scope of the present disclosure.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the described subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present description. These are, of course, merelyexamples and are not intended to be limiting. In addition, the presentdisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition is for the purpose of simplicity and clarityand does not in itself dictate a relationship between the variousembodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

“Vertical direction” and “horizontal direction” are to be understood asindicating relative directions. Thus, the horizontal direction is to beunderstood as substantially perpendicular to the vertical direction andvice versa. Nevertheless, it is within the scope of the presentdisclosure that the described embodiments and aspects may be rotated inits entirety such that the dimension referred to as the verticaldirection is oriented horizontally and, at the same time, the dimensionreferred to as the horizontal direction is oriented vertically.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

Embodiments in accordance with the present disclosure provide methodsand systems for removing impurities from a liquid fuel that is suppliedto a droplet generator of an EUV photolithography system. If notremoved, the impurities can negatively affect the ability of the DGA tostably generate droplets. If the DGA is not able to generate droplets ina stable manner, the quality of the pattern features degrades.Embodiments in accordance with the present disclosure concentrate theimpurities near an interface between the liquid fuel and a gaseousheadspace in the vessel containing the liquid fuel. Impurities areremoved from the vessel, e.g., by drawing them into a suction conduit.In accordance with some embodiments, the distance the suction conduitpenetrates into the liquid fuel is controlled such that the inlet of thesuction conduit is not too far below the location where the impuritiesare concentrated. When the suction conduit penetrates into the liquidfuel such that the inlet of the suction conduit is too far below thelocation where the impurities are concentrated, drawing the impuritiesinto the suction conduit becomes more challenging.

FIG. 1 is a block diagram of a photolithography system 100. Thephotolithography system 100 includes a laser 102, a photolithographytarget 105, a collector 106, a droplet generator 108, and a dropletreceiver 110. The droplet receiver 110 includes a droplet pool (notshown in FIG. 1 ) in which the droplets received by the droplet receiveraccumulate. The components of the photolithography system 100 cooperateto generate extreme ultraviolet (EUV) radiation which is used in an EUVphotolithography process to pattern materials on the photolithographytarget.

The droplet generator 108 generates and outputs a stream of droplets.The droplets can include, in one example, liquid (melted) tin. Othermaterials can be used for the droplets without departing from the scopeof the present disclosure. The droplets move at a high rate of speedtoward the droplet receiver 110. The photolithography system 100utilizes the droplets to generate extreme ultraviolet light forphotolithography processes. Extreme ultraviolet light typicallycorresponds to light with wavelengths between 5 nm and 125 nm.

The laser 102 outputs a laser beam. The laser beam is focused on a pointthrough which the droplets pass on their way from the droplet generator108 to the droplet receiver 110. In particular, the laser 102 outputslaser pulses. Each laser pulse is received by a droplet. When thedroplet receives one or more of the laser pulses, the energy from thelaser pulses generates a high-energy plasma from the droplet. Thehigh-energy plasma outputs extreme ultraviolet radiation.

In one embodiment, the radiation output by the plasma scatters randomlyin many directions. The photolithography system 100 utilizes thecollector 106 to collect the scattered extreme ultraviolet radiationfrom the plasma droplets and reflect the extreme ultraviolet radiationtoward a photolithography target 105, or toward equipment that willguide the extreme ultraviolet radiation to the photolithography target105.

In one embodiment, the collector 106 includes an aperture. The laserpulses from the laser 102 pass through the aperture toward the stream ofdroplets. This enables the collector 106 to be positioned between thelaser 102 and the photolithography target 105.

After the droplets have been irradiated by the laser 102, the dropletscontinue with a trajectory toward the droplet receiver 110. The dropletreceiver 110 receives the droplets. The droplets can be drained from thedroplet receiver 110 and reused or disposed of.

FIG. 2 is a more detailed illustration of a photolithography system 201,according to an embodiment of the present disclosure. Thephotolithography system 201 includes a laser 102, a collector 106, adroplet generator 108 including a droplet generator nozzle 109, adroplet receiver 110 including a droplet pool 116, a source of dropletliquid 120 and a source of an inert gas 118.

In the illustrated embodiment, the droplet generator 108 generates andoutputs a stream of droplets 124. The droplets are formed by driving adroplet liquid through a source of droplets, e.g., a droplet generatornozzle 109 of the droplet generator 108. The droplet liquid is suppliedto the droplet generator 108 from a source of droplet liquid 120. Thedroplet liquid delivered to the droplet generator 108 is pressurized todrive the droplet liquid through the nozzle 109. In one embodiment, thesource of droplet liquid 120 is in fluid communication with the firstsource of inert gas 118, e.g., argon. The inert gas exerts a pressure onthe droplet liquid that is fed from the source of droplet liquid 120 tothe droplet generator 108. In other embodiments, the pressure exerted onthe droplet liquid is supplemented by a mechanical device or energy. Inother embodiments, the pressure exerted on the droplet liquid isprovided by a mechanical device or energy. The droplets can include, asdescribed previously, tin. The droplets 124 ejected from nozzle 109 moveat a high rate of speed toward the droplet receiver 110. The rate atwhich the droplets 124 are generated by droplet generator 108 iscontrolled and coordinated with pulsing of the laser 102 such that asmany droplets as possible are irradiated to generate the plasma whichgenerates the EUV radiation. The droplet generator nozzle 109 ejects thedroplets such that the droplets have X, Y and Z direction coordinatesthat cause as many, if not all, of the droplets to be received by thedroplet receiver 110, such that the droplets do not impinge upon thereflective surface of collector 106 or other surfaces of thephotolithography system 201 where deposition of the droplets is notdesired.

The laser 102 is positioned behind a collector 106. The laser 102outputs pulses of laser light 132. The pulses of laser light 132 arefocused on a point through which the droplets pass on their way from thedroplet generator nozzle 109 to the droplet receiver 110. Each pulse oflaser light 132 is received by a droplet 124. When the droplet 124receives one or more of the pulses of laser light 132, the energy fromthe laser pulses generates a high-energy plasma from the droplet 124.The high-energy plasma outputs extreme ultraviolet radiation.

In one embodiment, the laser 102 is a carbon dioxide (CO2) laser. TheCO2 laser emits radiation or laser light 132 with a wavelength centeredaround 9.4 μm or 10.6 μm. The laser 102 can include lasers other thancarbon dioxide lasers and can output radiation with other wavelengthsthan those described above without departing from the scope of thepresent disclosure.

In one embodiment the droplet generator 108 generates between 40,000 and60,000 droplets per second. The droplets 124 have an initial velocity ofbetween 70 m/s and 90 m/s. The droplets have a diameter between 10 μmand 200 μm. The droplet generator 108 can generate different numbers ofdroplets per second than described above without departing from thescope of the present disclosure. The droplet generator 108 can alsogenerate droplets having different initial velocities and diameters thanthose described above without departing from the scope of the presentdisclosure.

In one embodiment, the laser 102 irradiates each droplet 124 with twopulses. A first pulse causes the droplet 124 to flatten into a disk-likeshape. The second pulse causes the droplet 124 to form a hightemperature plasma. The second pulse is significantly more powerful thanthe first pulse. The laser 102 and the droplet generator 108 arecalibrated so that the laser 102 emits pairs of pulses such that eachdroplet 124 is irradiated with a pair of pulses. For example, if thedroplet generator 108 outputs 50,000 droplets per second, the laser 102will output 50,000 pairs of pulses per second. The laser 102 canirradiate droplets 124 in a manner other than described above withoutdeparting from the scope of the present disclosure. For example, thelaser 102 may irradiate each droplet 124 with a single pulse or withmore pulses than two.

In one embodiment, the droplets 124 are tin. When the tin droplets 124are converted to a plasma, the tin droplets 124 output extremeultraviolet radiation 134 with a wavelength centered between 10 nm and15 nm. More particularly, in one embodiment the tin plasma shines with acharacteristic wavelength of 13.5 nm. These wavelengths correspond toextreme ultraviolet radiation. Materials other than tin can be used forthe droplets 124 without departing from the scope of the presentdisclosure. Such other materials may generate extreme ultravioletradiation with wavelengths other than those described above withoutdeparting from the scope of the present disclosure.

In one embodiment, the radiation 134 output by the droplets scattersrandomly in many directions. The photolithography system 100 utilizesthe collector 106 to collect the scattered extreme ultraviolet radiation134 from the plasma and output the extreme ultraviolet radiation towarda photolithography target 105.

In one embodiment, the collector 106 is a parabolic or ellipticalmirror. The scattered radiation 134 is collected and reflected by theparabolic or elliptical mirror with a trajectory toward aphotolithography target 105.

In one embodiment, the collector 106 includes an aperture 135. Thepulses of laser light 132 pass from the laser 102 through the aperture135 toward the stream of droplets 124. This enables the collector 106 tobe positioned between the laser 102 and the photolithography target 105.

After the droplets 124 have been irradiated by the laser 102, thedroplets 124 continue with a trajectory toward the droplet receiver 110.In particular, the droplets enter the droplet receiver 110 and travelthrough an interior passage toward a droplet pool 116 at a back end ofthe droplet receiver 110. The droplet pool 116 collects the droplets124. The droplet receiver 110 can further include a drain port (notshown) that drains the droplet pool 116. The droplets 124 can be reusedor disposed of.

FIG. 3 shows in greater detail a portion of a droplet generator 108 forgenerating droplets. For the generalized embodiment shown in FIG. 3 ,droplet generator 108 includes a reservoir 121 holding a molten dropletliquid material such as tin. Heating elements (not shown) controllablymaintain portions of the droplet generator 108 at a temperature abovethe melting temperature of the material comprising the droplet liquid.The molten droplet material may be placed under pressure by using aninert gas such as argon from the inert gas source 118 in FIG. 2 via line96. The pressure preferably forces the molten droplet material to passthrough a set of filters 98. From the filters 98, the material may passthrough a valve 101 to a nozzle 109. For example valve 101 may be athermal valve. A Peltier device may be employed to establish the valve101, freezing target material between the filters 98 and nozzle 109 toclose the valve 101 and heating the solidified target material to openthe valve 101. FIG. 3 also shows that the droplet generator 108 includesa movable member 104 such that motion of the movable member 104 changesthe position of the point at which droplets are released from the nozzle109. Motion of the movable member 104 is controlled by a droplet releasepoint positioning system (not shown). The inert gas is not limited toargon and maybe helium or nitrogen or another gas that does not reactwith tin to form oxides of tin or does not react with the nozzle to formoxides of the material from which the nozzle 109 is manufactured.

For the droplet generator 108, one or more modulating or non-modulatingdroplet liquid dispensers may be used. For example, a modulatingdispenser may be used having a capillary tube formed with an orifice.The nozzle 109 may include one or more electro-actuatable elements,e.g., actuators made of a piezoelectric material, which can beselectively expanded or contracted to deform the capillary tube andmodulate a release of source material from the nozzle 109.

As stated, the droplets are released by a nozzle 109. To be useful as anozzle, the nozzle preferably is able to operate at relatively highpressures, for example, from about 6000 pounds per square inch to about8000 pounds per square inch. It is also preferable that the nozzlepermit good control over the exit angle and velocity of the droplets. Itis also preferable that the nozzle enable flexibility in permittingmultiple design options for coupling the nozzle to other components inthe system, in particular, to elements that are provided to modulate thedroplet stream.

FIG. 4 illustrates delivery system 200 for delivering fuel, e.g., moltentin, to a DGA 210 of the present disclosure that is useful in an EUVlithography system. The delivery system 200 includes a refill andpriming assembly (RPA) 202, a liquid (e.g., molten tin) refill assembly(TRA) 204, a liquid fuel (e.g., molten tin) storage or supply assembly(TSA) 206, a liquid fuel (e.g., molten tin) transfer assembly (TTA) 208,and a droplet generator assembly (DGA) 210 with nozzle 234. When the DGA210 is discharging or ejecting a liquid fuel 211, the liquid fuel 211moves through the various components of the fuel delivery system 200 ina direction along the fuel delivery system 200 from the TRA 204 to theDGA 210. The liquid fuel 211 is the same or similar to the molten tin asdiscussed earlier in the present disclosure with respect to FIGS. 1-3 .

The RPA 202 is coupled to and in fluid communication with the TRA 204.The RPA 202 is configured to be utilized to convert a solidified fuelinto a liquid phase. For example, a solidified fuel (e.g., solid tin) isplaced within a heating element or component (e.g., heating container)212 that heats up the solidified fuel causing a phase change from thesolidified fuel to the liquid fuel 211. The liquid fuel 211 then movesalong the RPA 202 to a filter/tank 214 that filters the liquid fuel 211to remove contaminants or impurities present within the liquid fuel 211that may still be solid phase and stores the filtered liquid fuel. Allthe filter is effective in removing some contaminants or impurities fromthe liquid fuel, some are not removed by the filter or form in theliquid fuel after it has passed through the filter. For example, oxidesof the liquid fuel may form in the liquid fuel as well as bubbles ofgas. Both of these impurities can negatively impact the stability ofdroplets formed by the DGA 210 utilizing a liquid fuel that includessuch impurities. When there is a demand for the filtered liquid fuel211, the liquid fuel 211 enters a first end 216 of the TRA 204 andpasses through the TRA 204 to a second end 218 of the TRA 204. Theliquid fuel 211 then enters the TSA 206. The TRA 204 may be a pipe.

After the liquid fuel 211 passes through the second end 218 of the TRA204, the liquid fuel 211 enters the TSA 206. The liquid fuel 211 isstored in at least one liquid fuel container or vessel of the TSA 206.In this embodiment, the TSA 206 includes a first liquid fuel container220 and a second liquid fuel container 222. The liquid fuel 211 isstored in the first and second containers 220, 222, and the first andsecond containers 220, 222 may be opened and closed in a controlledmanner to limit an amount of liquid fuel 211 provided to the TTA 208.For example, the amount of liquid fuel 211 introduced into the TTA 208may be controlled by controlling opening and closing of a plurality ofvalves 224, 226 in fluid communication with the first and second liquidfuel containers 220, 222, respectively. In this embodiment, theplurality of valves 224, 226 includes a first valve 224 in fluidcommunication between the TSA 206 and the TRA 204 and a second valve 226in fluid communication between the TSA 206 and the TTA 208. The TTA 208may be a pipe.

Limiting the amount of the liquid fuel 211 provided to the TTA 208limits and controls the amount of the liquid fuel 211 introduced to theDGA 210 to avoid damaging the DGA 210. For example, if too much of theliquid fuel 211 is introduced to the DGA 210, the DGA 210 may not beable to eject or discharge the liquid fuel 211 with enough rapidness toavoid the liquid fuel 211 from overflowing from the DGA 210 or from alarge pressure building up within the DGA 210. The overflowing of theDGA 210 or the large build of pressure within the DGA 210 may causeundue stress and strain to components with the DGA 210, which may reducethe useful life span of the DGA 210 or result in failure of the DGA 210due to components breaking or failing (e.g., breaking, cracking,shearing).

When the liquid fuel 211 is introduced to the TTA 208, the liquid fuel211 enters a first end 228 of the TTA 208 and passes through the TTA 208to exit at a second end 230 of the TTA 208 such that the liquid fuel 211is introduced to the DGA 210 through a DGA valve 232. Under a normaloperation (which is similar to the normal operation as discussed earlierwith respect to FIG. 1 ), when the liquid fuel 211 enters the DGA 210through the DGA valve 232, the liquid fuel 211 moves through the DGA 210to a nozzle 234 (which is the same or similar to the nozzle 109 of theDGA 210 as discussed earlier with respect to FIGS. 2 and 3 ). The liquidfuel 211 is then ejected or discharged from the DGA 210 through thenozzle 234 in the form of stream of discrete droplets of the liquid fuel211 or a continuous stream of the liquid fuel 211. The liquid fuel 211may be ejected or discharged from the DGA 210 into a vacuum chamber,and, when in the vacuum chamber, the liquid fuel 211 is exposed to alaser generating EUV light.

Nozzle 109 ideally has a cross-section perpendicular to the flow ofliquid fuel through nozzle 109 that is perfectly round with perfectlysmooth surfaces; however, nozzle 109 may not be perfectly round and maynot include perfectly smooth surfaces. When nozzle 109 is not perfectlyround and does not include perfectly smooth surfaces, impurities withinthe liquid fuel are more prone to depositing or collecting on surfacesof the nozzle. Such collection of impurities can negatively impact theability of the nozzle to produce droplets in a stable manner.

Referring to FIG. 5 , an enlarged view of a vessel 500 containing aliquefied fuel 211, e.g., liquefied tin, in accordance with anembodiment of the present disclosure is illustrated. Vessel 500illustrated in FIG. 5 maybe the filter/tank 214 of RPA 202 in FIG. 4 orthe vessels 220 and 222 of TSA 206 in FIG. 4 or other vessel of fueldelivery system 200 of FIG. 4 . Also illustrated in FIG. 5 is animpurity tank 502, pump 504, suction conduit 506, level sensor 508, gasconduit and valve 511. FIG. 5 also illustrates additional features inaccordance with an embodiment of the present disclosure, including gasconduit 510 and valve 511. In other embodiments, vessel 500 includesliquid fuel circulation conduit 514 and valve 516. In FIG. 5 , analternative to sensor 508 is illustrated as sensor 518 positioned on asidewall 519 of vessel 500.

In FIG. 5 , in accordance with embodiments of the present disclosure,liquid fuel 211 includes a plurality of impurities 512, which in FIG. 5, are illustrated as having collected or concentrated near an uppersurface 513 of liquid fuel 211. Upper surface 513 is located at aninterface between liquid fuel 211 and gas located above liquid fuel 211within vessel 500. It is understood that in accordance with embodimentsof the present disclosure, impurities 512 may not be concentrated asillustrated in FIG. 5 until after methods in accordance with the presentdisclosure are implemented.

Impurities 512 can be any material that negatively impacts the abilityof DGA 210 to stably reproduce droplets used to generate extremeultraviolet radiation. For example, impurities 512 may be oxides of thefuel, e.g., tin oxide when the fuel is tin. The impurities 512 may bebubbles of gas in liquid fuel 211. In other embodiments, the impuritiesmay be materials present in the solid fuel which remain solid after thesolid fuel is liquefied, i.e., melted.

In FIG. 5 , impurity tank 502 is in fluid communication with pump 504which is in fluid communication with suction conduit 506. An end ofsuction conduit 506 opposite pump 504 is positioned below upper surface513 of liquid fuel 211 such that impurities located below the uppersurface 513 can be drawn into suction conduit 506. Impurities locatedbelow the upper surface 513 are drawn into suction conduit 506 throughoperation of pump 504 and delivered to impurity tank 502 where theimpurities can be separated from the liquid fuel. In accordance withembodiments of the present disclosure, when impurities located belowupper surface 513 of liquid fuel 211 are drawn into suction conduit 506,some portion of liquid fuel 211 may also be drawn into suction conduit506. In accordance with the embodiment illustrated in FIG. 5 , a sensor508 is attached to suction conduit 506 at a location that is betweenupper surface 513 of liquid fuel 211 and the lower surface 515 of a top517 of vessel 500. In one embodiment, sensor 508 is a contact sensorwhich detects when liquid fuel 211 comes in contact with contact sensor508. In an alternative embodiment, sensor 508 is a proximity sensor suchas an infrared sensor that is able to detect a distance between sensor508 and an upper surface 513 of liquid fuel 211. While sensor 508 isillustrated as being attached to suction conduit 506, in otherembodiments, a sensor 518 (has capabilities similar to sensor 508) canbe located on a sidewall 519 of vessel 500. In another embodiment,sensor 508 can be located on the lower surface 515 of the top 517.

Sensors 508 or 518 are utilized to detect the location of upper surface513 of liquid fuel 211. The information regarding the location of uppersurface 513 of liquid fuel 211 can be compared to a threshold value andused to control the flow of liquid fuel 211 into or out of vessel 500 sothat end of suction conduit 506 does not penetrate too deeply intoliquid fuel 211. When the end of suction conduit 506 penetrates toodeeply into liquid fuel 211, the opening in the end of suction conduit506 through which the impurities flow can be below the concentration ofimpurities. When the opening is below the concentration of impurities,the likelihood that such impurities will be sucked into the suctionconduit 506 when pump 504 is activated decreases. For example, if thelocation of upper surface 513 is above the threshold value, thusindicating that the opening in the end of suction conduit 506 ispenetrating too deeply into the liquid fuel, steps are taken to changethe degree to which the end of suction conduit 506 penetrates into theliquid fuel. For example, the net flow of liquid fuel into vessel 500can be reduced. Alternatively, when the location of upper surface 513 isabove the threshold value, operation of pump 504 can be stopped suchthat liquid is not removed from the vessel 500. On the other hand, ifthe location of upper surface 513 is below the threshold value, thusindicating that the opening in the end of suction conduit 506 is notpenetrating too deeply into liquid fuel, or may not be penetratingdeeply enough, steps are taken to increase the degree to which the endof suction conduit 506 penetrates into the liquid fuel. For example, thenet flow of liquid fuel into vessel 500 can be increased.

In accordance with embodiments of the present disclosure, due to thelower density of the impurities compared to the density of the liquidfuel, the impurities tend to rise within liquid fuel. For example, insome embodiments, the impurities are located in the upper ½ of theliquid fuel 211. In other embodiments, the impurities are located in theupper ¼ of the liquid fuel 211. In other embodiments, the impurities arelocated within about 1 to about 30 mm of the upper surface 513 of liquidfuel 211.

In FIG. 5 , an optional suction conduit actuator 520 is illustrated.Optional suction conduit actuator 520 provides an alternative tocontrolling the penetration of suction conduit 506 into liquid fuel 211.Suction conduit actuator 520 is connected to top 517 of vessel 500 andis connected to suction conduit 506. In operation, suction conduitactuator 520 changes the location of the end of suction conduit 506relative to the top 517 of vessel 500. For example, suction conduitactuator 520 can lower the end of suction conduit 506 relative to thetop 517 of vessel 500 or it may raise the end of suction conduit 506relative to the top 517 of vessel 500. In this manner, suction conduitactuator 520 is able to maintain the end of suction conduit 506 in theportion of the liquid fuel 211 where the impurities are collecting orconcentrating. Suction conduit actuator 520 may be powered by a servomotor or other drive mechanism capable of precisely positioning the endof suction conduit 506 relative to the upper surface 513 of liquid fuel211.

The system illustrated in FIG. 5 also includes a gas conduit 510 influid communication with the headspace 522 in vessel 500. In accordancewith embodiments of the present disclosure, gas conduit 510 is utilizedto deliver an inert gas to headspace 522 in order to prevent or reduceoxidation of liquid fuel in vessel 500. Gas conduit 510 includes a valve511 for controlling the flow of an inert gas, e.g., argon or hydrogengas, into headspace 522. Gas conduit 510 can also be in fluidcommunication with a vacuum source so that a vacuum can be pulled on theheadspace 522. For example, a positive pressure can be provided inheadspace 522 by delivering gas to headspace, a negative (i.e., vacuum)pressure can be provided in headspace 522 by pulling a vacuum onheadspace 522 or alternating positive pressures and negative pressurescan be provided in headspace 522. In accordance with some embodiments ofthe present disclosure, decreasing the pressure in headspace 522, maypromote the rising of the gas bubbles through the liquid fuel or promotegases dissolved within the liquid fuel to come out of the liquid fuel.In other embodiments of the present disclosure, alternating the pressurefrom a positive pressure to a negative pressure (e.g., a vacuumpressure) or a higher pressure and a lower pressure in headspace 522 maypromote the rising of impurities through the liquid fuel.

The system illustrated in FIG. 5 also includes an optional liquid fuelcirculation conduit 514 that includes an inlet 524 and an outlet 527,both in fluid communication with the interior of vessel 500. Liquid fuelcirculation conduit 514 includes a valve 516 and pump 526 forcontrolling flow of liquid fuel 211 through liquid fuel circulationconduit 514. In accordance with embodiments of the present disclosure,liquid fuel circulation conduit 514, valve 516 and pump 526 are utilizedto circulate liquid fuel out of and back into vessel 500 in order tocreate currents, e.g., vortices, within liquid fuel 211 in vessel 500that promote the concentration of impurities 512 at a location wherethey can be removed from vessel 500 via suction conduit 506.

The embodiment illustrated in FIG. 5 includes an optional agitationsubsystem 540 for agitating the liquid fuel. Agitation subsystem 540 caninclude devices capable of agitating the liquid fuel. For example,agitation subsystem 540 can include a stirrer for agitating the liquidfuel. In another example, agitation subsystem 540 includes a source ofsonic energy which can be directed into the liquid fuel or which cancause vibration or agitation of the vessel 500 which would have theeffect of agitating liquid fuel within vessel 500.

Referring to FIG. 6 , embodiments in accordance with methods of thepresent disclosure can be implemented in other types of vessels besidesa vessel of an RPA or TSA. For example, as illustrated in FIG. 6 ,methods of the present disclosure can be implemented in a fuel reservoir600 of a DGA 210 in FIG. 4 or 108 in FIGS. 1-3 . The fuel reservoir 600of a DGA is in fluid communication with a nozzle 602 through which thefuel flows to generate droplets. Nozzle 602 is in fluid communicationwith reservoir 600 through the conduit 604. Nozzle 602 is associatedwith at least one filter and one valve, schematically illustratedtogether as 606. Features illustrated in FIG. 6 , to which thedescriptions of FIG. 5 are applicable are identified by the samereference numerals used in FIG. 5 . The embodiment illustrated in FIG. 6, includes an impurity tank 502, a pump 504 and a suction conduit 506.As in the embodiment of FIG. 5 , an end of suction conduit 506 oppositeimpurity tank 502 is positioned below the surface 513 of liquid fuel 211where it can draw impurities 512 into suction conduit 506. Impurities512 include gas bubbles 614 and non-gas impurities 616, e.g., solidparticles such as oxides of the liquid fuel. The embodiment of FIG. 6also includes a source 618 of inert gas, e.g., such as argon or hydrogenthat is used to provide pressure which drives the liquid fuel intoconduit 604. As with the embodiment described above with reference toFIG. 5 , operation of pump 504 draws impurities 512 into suction conduit506 and thereby prevents the impurities 512 from negatively impactingthe ability of nozzle 602 to generate droplets in a stable andreproducible manner. Though not illustrated in FIG. 6 , the embodimentof FIG. 6 can include sensors similar to sensors 508 and 518 describedwith reference to FIG. 5 . The embodiment of FIG. 6 can also includesuction conduit actuator 520 and liquid fuel circulation conduit 514,including valve 516 and pump 526.

FIG. 7 is a flow diagram of a method 700 for processing a fuel of an EUVphotolithography system. At 720, the method 700 includes collecting aliquefied solid fuel in a vessel. One example of collecting a liquefiedfuel includes heating solid fuel in heating component 212 of refill andpriming assembly RPA 202 and flowing the liquefied fuel to filter/tank214 where it will be collected. At 730, impurities in the liquefied fuelcontained in the filter/tank 214 are collected or concentrated asdescribed above with reference to FIGS. 5 and 6 . An example ofconcentrating the impurities in the liquefied fuel includes allowing theimpurities to rise through the liquefied fuel. Impurities such as oxidesof the liquid fuel or gas bubbles within the liquid fuel have a densitythat is less than the density of the liquid fuel and thus, are able torise through the liquid fuel. In some embodiments, the impurities can beconcentrated by agitating the liquefied fuel as described above or bycreating currents or vortices within the liquefied fuel as describedabove. At 740, the concentrated impurities 512 are removed from thevessel 500. An example of removing impurities 512 from the vessel 500includes suctioning the impurities out of the vessel through suctionconduit 506 as described above with reference to FIG. 5 . In accordancewith some embodiments of the method illustrated in FIG. 6 , removal ofthe impurities 512 from vessel 500 includes removing a portion ofliquefied fuel from vessel 500.

FIG. 8 is a flow diagram of another method 800 in accordance withembodiments described herein for removing impurities from a liquid fuelused to generate droplets in an EUV photolithography system. At 810, asolid fuel is liquefied. An example of how a solid fuel is liquefiedincludes heating a solid fuel in heating component 212 of refill andpriming assembly RPA 202. At 820, the method 800 includes collecting theliquefied solid fuel in a vessel. One example of collecting a liquefiedfuel in a vessel includes flowing the liquefied fuel from heatingcomponent 212 to filter/tank 214 where it is collected. At 830,impurities 512 in the liquefied fuel 211 contained in the filter/tank214 are allowed to rise through the liquefied fuel. As discussed above,the impurities 512 rise through the liquefied fuel 211 because thedensity of the impurities is less than the density of the liquefiedfuel. As the impurities rise through the liquefied fuel, they collectand concentrate near the interface 513 between the liquefied fuel 211and the gas containing headspace 522 in vessel 500. At 840, collected orconcentrated impurities 512 are removed from vessel 500 via suctionconduit 506 as described above with reference to FIG. 5 .

In one embodiment in accordance with the present disclosure, a method ofprocessing fuel in an EUV photolithography system includes forming avolume of liquid fuel in a vessel. The liquid fuel includes impuritieswhich are concentrated in the liquid fuel. After the impurities in theliquid fuel have concentrated, they are removed from the vessel.Removing the impurities from the liquid fuel increases the likelihoodthat a DGA fed with such cleansed fuel will be able to generate dropletsstably and for an extended period of time as compared to a DGA suppliedwith liquid fuel that has not had the impurities removed. Increasing theability of the DGA to generate droplets stably for an extended period oftime decreases the number of wafers due to poor photolithographicpatterning and increases the length of time between regular maintenanceof the DGA and/or replacement of the DGA.

In another embodiment, a method in accordance with the presentdisclosure includes liquefying a solid fuel and collecting the liquefiedsolid fuel in a vessel. In accordance with this embodiment, impuritiesin the liquefied solid fuel are allowed to rise towards an interfacebetween the liquefied solid fuel in the vessel and the gas in thevessel. The impurities which have risen towards an interface between theliquefied solid fuel in the vessel and the gas in the vessel are removedfrom the vessel.

An embodiment of a method of purifying a fuel in an EUV photolithographysystem in accordance with the present disclosure includes collecting aliquefied tin fuel in a vessel. Impurities in the liquefied tin fuel arecaused to rise through the liquefied tin fuel, e.g., by agitating theliquefied tin fuel or reducing the pressure above the liquefied tin fuelconcentrates. The rising impurities concentrate before being removedfrom the vessel with a portion of the liquefied tin fuel.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary, to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A method of processing fuel in an EUVphotolithography system, the method comprising: forming a volume ofliquid fuel in a vessel, the liquid fuel including impurities;concentrating the impurities in the liquid fuel; and suctioning theimpurities out of the liquid fuel.
 2. The method of claim 1, wherein theconcentrating the impurities in the liquid fuel includes concentratingone or more of oxides of the liquid fuel and bubbles of gas in theliquid fuel.
 3. The method of claim 1, wherein the liquid fuel is moltentin.
 4. The method of claim 1, wherein the concentrating the impuritiesin the liquid fuel includes allowing the impurities to rise through theliquid fuel.
 5. The method of claim 1, wherein the concentrating theimpurities in the liquid fuel includes generating a fluid flow patternin the vessel that concentrates the impurities.
 6. The method of claim5, wherein generating a flow pattern in the vessel that concentrates theimpurities includes generating a vortex within the liquid fuel.
 7. Themethod of claim 1, wherein the impurities are suctioned out of theliquid fuel via a suction tube.
 8. The method claim 7, wherein thesuctioning of impurities out of the liquid fuel via the suction tubeceases when a level of the liquid fuel in the vessel rises above athreshold level.
 9. The method claim 1, further comprising altering adistance that a suction tube penetrates into the liquid fuel.
 10. Themethod of claim 1, further comprising ceasing the suctioning of theimpurities out of the liquid fuel when a level of the liquid fuel risesabove a threshold level.
 11. A method comprising: liquefying a solidfuel; collecting the liquefied fuel in a vessel; allowing impurities inthe liquefied fuel to rise to an interface between the liquefied fuel inthe vessel and a gas in the vessel; adjusting a distance a suction tubepenetrates into the liquified fuel; and removing the impurities from thevessel.
 12. The method of claim 11, further comprising ceasing theremoving the impurities from the vessel when a level of the liquefiedfuel rises above a threshold level.
 13. The method of claim 12, furthercomprising determining the level of the liquefied fuel has risen abovethe threshold level utilizing a sensor configured to detect a surface ofthe liquefied fuel.
 14. The method of claim 12, wherein the liquefiedfuel is tin.
 15. The method of claim 14, wherein the impurities areoxides of tin.
 16. The method of claim 11, further comprising ceasingthe removing from the vessel, a portion of the liquified tin fuelcontaining the concentrated impurities when a level of the liquified tinfuel rises above a threshold level.
 17. A method of purifying a fuel inan EUV photolithography system, the method comprising: collecting a tinfuel in a liquid phase in a vessel; causing impurities in the tin fuelin a liquid phase to rise through the tin fuel in a liquid phase;concentrating the impurities in the tin fuel in a liquid phase; andremoving from the vessel, a portion of the tin fuel in a liquid phasecontaining the concentrated impurities.
 18. The method of claim 17,wherein the causing impurities in the tin fuel in a liquid phase to risethrough the tin fuel in a liquid phase includes agitating the tin fuelin a liquid phase.
 19. The method of claim 17, wherein the causingimpurities in the tin fuel in a liquid phase to rise through the tinfuel in a liquid phase includes reducing pressure above the tin fuel ina liquid phase.
 20. The method of claim 19, wherein the causingimpurities in the tin fuel in a liquid phase to rise through the tinfuel in a liquid phase includes alternating between a positive pressureand a vacuum pressure.